1. Field of the Invention
The present invention relates to spatial light modulators and more specifically to a Digital Micromirror Device (DMD) spatial light modulator utilizing a new noncontacting, edge-coupled hidden hinge geometry.
2. Description of the Related Art
FIG. 1 shows a conventional hidden hinge Digital Micromirror Device (DMD) broken into it""s three metal layers of construction, all of which is built on top of a rather standard SRAM memory cell. FIG. 1a shows the first metal layer 200 above the memory cell which consists of yoke address pads 1, a bias/reset bus 2, landing sites 3, and via connections 13 to the SRAM memory cell below (not shown). FIG. 1b shows the second layer 300 of metal structures consisting of the mirror address electrodes 4, electrode support posts 5, torsion hinge 6, hinge support posts 7, yoke 8, and the yoke landing tips 9. This second metal layer 300 assembly sits on top of the first metal layer 200, being supported by means of the electrode support posts 5 and hinge support post 7. Finally, FIG. 1c shows the third layer of metal 400 that consists of the highly reflective mirror 10 and it""s support post 11. As before, the mirror assembly 400 sits on top of the second metal layer 300, being supported by the mirror support post 11 sitting in the middle of the yoke 8. In operation, a bias voltage is applied to the bias/reset bus 2 that is integral to the yoke assembly 8 by means of the hinge support posts 7. The yoke address pads 1 and mirror address electrodes 4 are then pulsed to establish an electric field between the address pads and the mirror assembly that generates an electrostatic force causing the yoke/mirror assembly to tilt in one direction or the other depending on the binary state of the underlying memory cell. As illustrated, although the yoke and mirror assemblies rotate together, electrostatic forces are established in two areas 12 (shown as cross hatched areas); i.e., between the yoke address pad 1 on the first level and the yoke 8 on the second level, as well as between the mirror address electrodes 4 on the second level and the mirror 10 on the third level. The yoke 8 rotates until its two landing tips 9 contacts the landing sites 3 on the lower metal layer 200. The angle of rotation is a function of the yoke geometry and the height of the second metal 300 layer above the lower metal layer 200. The long, thin, narrow torsion hinges 6, which supports the yoke 8 and mirror 10 from the hinge support posts 7, have a torque applied to them allowing the thicker yoke 8 to remain flat. Finally, a reset pulse can be applied to the bias/reset bus 2 to lift off and free the mirror/yoke assembly from the landing sites 3.
FIG. 2 shows a three-dimensional build-up of a conventional DMD""s four layers, including the SRAM memory, which was mentioned above. These consist of the CMOS SRAM memory layer 100, the address and landing pad layer 200, the yoke and hinge layer 300, and the mirror layer 400. It can be seen from the figure that this conventional DMD device is symmetrical about a diagonal axis running parallel with the hinge, so that in operation the mirror assembly will tilt in the positive or negative direction depending on the binary state (xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d) of the SRAM memory cell 14. The geometry of a typical DMD is such that the mirror will tilt on the order of xc2x1100.
FIG. 3 is a 3-D cutaway view of an array of conventional hidden hinge DMD pixels showing three of the mirrors and the underlying structure for other pixels. Included in the view are the following: yoke address pad 1, bias/reset bus 2, yoke landing sites 3, mirror address electrode 4, electrode support post 5, torsion hinge 6, hinge support posts 7, yoke 8, yoke landing tips 9, reflective mirror 10, mirror support post 11, vias 13 to SRAM memory cell 14. The square mirrors tilt on the order of xc2x110xc2x0 and are highly reflective to visual light in the color spectrum from 400 to 650 nanometers. The gaps between the mirrors are typically  less than 1 micron in width.
FIG. 4 illustrates two DMD cells, with their mirrors 10 shown transparent so as to expose a view of the underlying structure. One mirror is shown rotated xe2x88x9210xc2x0 and the other is shown rotated +10xc2x0, representing a xe2x80x9c0xe2x80x9d and xe2x80x9c1xe2x80x9d binary state, respectively in the underlying memory cells 14. This figure clearly shows how the yoke 8, with attached mirror 10, rotates on the torsion hinge 6 until the yoke landing tips 9 come in contact (lands) with the underlying landing pad sites 3. It is this mechanical contact between the yoke landing tips 9 and the landing pad sites 3 that is of particular relevance to this invention. A problem with conventional DMD""s is that of xe2x80x9csticking mirrorsxe2x80x9d, where the landing tips are slow in lifting off the pad, effecting the response of the device, or in some cases become permanently stuck to the landing pads. There appear to be several sources of this sticking problem, some of which include moisture in the package, landing tips scrubbing into the metal landing pads, and outgassing of the epoxy sealants used in the manufacturing process for mounting the devices in their packages and mounting the optical glass cover on the packages. This xe2x80x9cstickingxe2x80x9d problem has been addressed by applying a lubrication or passivation layer to the metal surfaces to make them xe2x80x9cslickxe2x80x9d and also through the use of resonant reset methods to pump energy into the pixel to help break it free from the constraining surface contact. More recently, xe2x80x9cspring-tipsxe2x80x9d have been added to the tips of the mirrors to help overcome this sticking problem. In addition, gettering material is often added to absorb moisture within the package. Although quite effective, these solutions still have the concern of long-term degradation of the passivant, which could drive the technology to a requirement for hermetic packages and complex process steps prior to window attachment. This would add additional expense, complexity, and difficulty in delivering the product.
It is therefore desirable to implement a DMD that will rotate reliably and predictably to a given angle, consistent across the length of the device or an array of pixels, without physically contacting the memory substrate surface below and as a result to avoid all the difficulties of breaking that contact. Eliminating the stiction problem would allow more predictable performance of the mirror array, and eliminate the most frequent cause of device failure; i.e., stuck bright mirrors. The lack of contact would also provide more immunity to particulates on the first electrode level, allow special dark metal light absorbing layers, and enable the use of conventional CMOS electrical passivation layers like SiO2. The invention disclosed herein addresses this need.
Representative prior conventional structures of the general type are shown in U.S. Pat. No. 5,535,047 to Hornbeck, and in publications (1) xe2x80x9cDigital Light Processing(trademark) for High-Brightness, High-Resolution Applications,xe2x80x9d by Larry J. Hornbeck, Electronic Imaging, EI""97, Projection Displays III, Co-Sponsored by ISandT and SPIE, Feb. 10-12 1997, san Jose, Calif., and (2) xe2x80x9cDigital Light Processing and MEMS: Timely Convergence for a Bright Future,xe2x80x9d Larry J. Hornbeck, Micromachining and Microfabrication ""95, Part of SPIE""s Thematic Applied Science and Engineering Series, Oct. 23-24 1995, Austin, Tex.
A new DMD device and method for non-contacting edge-coupled hidden hinge geometry is disclosed. This approach requires no physical contact between the mirror or underlying yoke and landing pads at the surface of the CMOS substrate. As a result, this eliminates the problem of xe2x80x9cstickingxe2x80x9d mirrors in conventional devices and significantly reduces the requirements for delicate passivation coatings and costly hermetic packages.
This method uses a more or less conventional DMD structure which still maintains digital operation, but one which has been modified to operate about either an orthogonal or diagonal axis, to deflect the pixel into the vicinity of the desired rotation position and then capture it with an edge coupled capacitive electrostatic force which latches the DMD mirror at the desired angle of rotation.
As with conventional DMD""s, the rotation angle is simply determined by the thickness of the organic layers used to build up the DMD superstructure. During fabrication of the device, a capture (positioning) electrode will be provided at an appropriate elevation above the surface of the device so as to be in close proximity with the continuous edge of the rotating mirror or yoke when it rotates into the region of the desired angle. This stop electrode is pulsed such as to establish a very high electrostatic attraction with the mirror assembly as it comes into the plane of the capture electrode. By tailoring the capture electrode pulse waveform, the mirror can be critically damped to provide stable operation and to prevent oscillations of the DMD mirrors about the desired angle of rotation.
This approach:
eliminates sticking due to contacting or landed DMD pixel architectures,
eliminates the need for passifying landing surface during manufacturing steps,
eliminates long term degradation due to landing surface contact and/or passivation degradation,
eliminates moisture sensitivity and therefore the need for hermetic packages, and
provides a fixed rotation angle through electrostatic capture, rather than a physical mechanical stop to determine rotational angle,
provides uniform release characteristic of mirrors upon release and return to flat state, and
enables a xe2x80x9cfast clearxe2x80x9d function where all mirrors can be sent to flat state by a single voltage change.